West Nile virus (WNV) is a pathogenic Flavivirus that has recently emerged in the Western hemisphere and has rapidly spread across the United States. As a result, cases of human infection with WNV have occurred in epidemic proportions and are increasingly associated with debilitating and fatal encephalopathy. Humans and other animals typically serve as dead-end hosts of WNV, which is transmitted through the bite of infected mosquitoes and is chiefly maintained in wild bird populations. The overwintering of infected mosquitoes has contributed to WNV gaining an endemic foothold within the immunologically naive U.S. population, thereby presenting an immediate and major public health risk. These properties of WNV and its ability to rapidly disseminate through bird-mosquito-human interactions mark it as a U.S. Centers for Disease Control and N.I.H. designated Category B pathogen and a potential agent of bioterrorism. Adequate response to the public health threat posed by WNV and other emerging Flaviviruses requires a thorough understanding of the virus-host interactions that support virus infection, replication and pathogenesis in human cells. This developmental project is focused on understanding the host response to WNV infection in human cells. We will characterize the host response to infection and will evaluate the application of interferon (IFN) as a therapeutic to limit WNV replication. Our studies will use three strains of lineage 1 WNV, including the current U.S. epidemic strain (WNV-NY), a related Texas isolate (WNV-TX) and the nonemergent Kunjin virus. In Aim 1 we will utilize a biochemical approach to define the mechanisms and distinctions of IFN action against the replication of each virus. In Aim 2 we will define the cellular processes that confer WNV-responsive gene expression in the host cell during the early stages of infection. Our preliminary studies have revealed that human cells respond to WNV virus by inducing the expression of a variety of antiviral response genes and that the administration of IFN can suppress WNV replication. This work will therefore identify the antiviral effectors that confer IFN action against WNV, and will define the host environment that confers infection susceptibility and control of virus replication among distinct isolates of WNV. Our studies will serve the N.I.H biodefense and emerging infectious disease research initiative.